The Collaborative International Dictionary of English v.0.48:

Athwart \A*thwart"\, prep. [Pref. a- + thwart.]
   1. Across; from side to side of.
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            Athwart the thicket lone.             --Tennyson.
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   2. (Naut.) Across the direction or course of; as, a fleet
      standing athwart our course.
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   Athwart hawse, across the stem of another vessel, whether
      in contact or at a small distance.

   Athwart ships, across the ship from side to side, or in
      that direction; -- opposed to fore and aft.
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The Collaborative International Dictionary of English v.0.48:

Athwart \A*thwart"\, adv.
   1. Across, especially in an oblique direction; sidewise;
      obliquely.
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            Sometimes athwart, sometimes he strook him straight.
                                                  --Spenser.
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   2. Across the course; so as to thwart; perversely.
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            All athwart there came
            A post from Wales loaden with heavy news. --Shak.
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WordNet (r) 3.0 (2006):

athwart
    adv 1: at right angles to the center line of a ship
    2: at an oblique angle; "the sun shone aslant into his face"
       [syn: obliquely, aslant, athwart]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "athwart":
   across, across the grain, adverse to, against, against the grain,
   against the tide, against the wind, at cross-purposes,
   at cross-purposes with, at daggers, at daggers drawn, at issue,
   at odds, at variance, at war with, athwartships, bendwise, beyond,
   bias, biased, biaswise, catercorner, catercornered, con, contra,
   contrariwise, contrawise, counter, counter to, crisscross, cross,
   cross-grained, crossway, crossways, crosswise, dead against,
   diagonal, eyeball-to-eyeball, in conflict with, in confrontation,
   in hostile array, in opposition, in opposition to, kittycorner,
   oblique, obliquely, opposed to, over, overthwart, sideways,
   sidewise, slant, thwart, thwartly, thwartways, transversal,
   transverse, transversely, traverse, up in arms, versus, vis-a-vis,
   with crossed bayonets